When Mahdi plants a dead tree, that tree will immediately be freshened and produce leaves.
Al-Qawl al-mukhtasar fi alamat al-Mahdi al-muntazar, 43
He (Mahdi) will plant a dead reed in a dry place, it will instantly get freshened and produce leaves.
Signs of the Judgment Day, 165
The Mahdi will plant a dry branch and this will sprout and produce leaves.
(Al-Burhan fi Alamat al-Mahdi Akhir al-zaman, p. 67)
An interpretation of these is as follows:
"It is meant that, through Mahdi's showing favor to and guiding a person, who is compared to a dead tree and who could not be of help to the people about him formerly just like a dead tree, such person would become beneficial to the people about him and to all the humanity just like a tree that has turned green and produced fruit. (Allah knows the truth)
The following hadith also points out in a similar way that a person, who was ignorant, mean, and coward formerly, would become a learned, generous and brave person, and change his personality just as a tree, which was dead and useless formerly, turns green and produces leaves, through guidance and instructions of the great spiritual guide of the end of time. (Allah knows the truth)
A man who is in fact ignorant, mean, and coward will instantly become wise, generous and brave.
Signs of the Judgment Day, 186
Imam Rabbani describes the power of teaching bestowed on him with the same analogy.
In matters of salvation, Almighty Allah has given me such great strength that if I plant a dry tree, it will immediately flourish.
Letters of Rabbani, 1/18
Call to the way of your Lord with wisdom and fair admonition, and argue with them in the kindest way. Your Lord knows best who is misguided from His way. And He knows best who are guided. (Surat an-Nahl: 125)
Remember when We made a covenant with the tribe of Israel: 'Worship none but Allah and be good to your parents and to relatives and orphans and the very poor. And speak good words to people. And establish salat and pay alms.' But then you turned away - except a few of you - you turned aside. (Surat al-Baqara: 83)
Go to Pharaoh; he has overstepped the bounds. But speak to him with gentle words so that hopefully he will pay heed or show some fear.' (Surah Ta Ha: 43-44)